Closure shells, such as crowns, lug or screw caps for bottles and jars, have been steadily improved with the growth of technological knowledge. Closures not only protect against contamination of the contents of bottles and jars, but also provide a sealing barrier to insure freshness of the products contained therein. In the past, closures have been provided with some sort of gasket or sealing material to provide a sealed volume within the containers, such as glass mason jars which were equipped with a separate rubber gasket which sealed the cap or top to the container portion of the jar. With the introduction of metallic closures, such as bottle caps or crowns, the gasket was replaced with a cork insert which covers the entire top interior of the crown so that the bottle was sealed and no metal could be contacted by the contents of the bottle. In this manner, the crown material could not adversely affect the contents of the bottle.
With the advance of chemical technology, the cork insert has gradually been replaced with a plastic insert, such as polyvinyl chloride (PVC). These plastic crown inserts are normally formed by two methods. In the first method, the polyvinyl chloride is molded while in the plastisol state by a one-piece plunger. This plunger seals against the sides or skirt of the crown and forms the plastisol into shape with a relatively small pressure exerted by the plunger, e.g., Ranier et al., U.S. Pat. No. 2,840,858.
In the second method, the inserts are formed from polyvinyl chloride while it is in a melt condition. In this method, a globule of PVC in some random shape is deposited in the crown and a plunger is introduced into the crown to press the PVC into the desired shape.
One apparatus for forming inserts according to this latter method is disclosed in Aichele, U.S. Pat. No. 3,135,019. The Aichele plunger consists of a rigid center punch of circular cross-sectional area which forms the central portion of the crown insert and causes the PVC to flow. Mounted concentrically with the plunger and axially slidable relative thereto is a spring loaded cushion or gasket forming die which is forced upwardly relative to the punch by the spreading PVC to form the gasket portion of the insert. Mounted coaxially with but outwardly of the cushion forming die and punch is a tapered fender sleeve which is introduced into the crown before the other components to form a sealed volume between the crown surface and the plunger components, thereby preventing the PVC from escaping while it is being compressed by the punch.
While the Aichele plungers produce acceptable inserts, the components of the punch, due to the very limited working space of the crown, necessarily require close dimensional tolerance. These plungers are costly to manufacture, not only due to their high tolerance requirements, but also due to the number of supporting components, i.e. precision loading springs for the cushion forming die and fender sleeves, as well as maintenance costs.
An alternate high-pressure insert plunger is disclosed in Schneider, U.S. Pat. No. 2,823,422, wherein a heated, one-piece, metallic plunger forms a crown insert. The Schneider plunger has a central forming surface, and an annular related portion joined with a peripheral rib. As the plunger is introduced into the crown, the central forming surface starts the thermoplastic material flowing; the rib then engages the corner radius of the crown to prevent flashing of the thermoplastic material from the bottom of the crown. As the plunger continues downwardly forming the insert, the rib forces the crown to deform, thus maintaining the seal and preventing flashing.
During the formation of thermoplastic inserts, two problems have plagued the industry. Since the plungers are generally mounted vertically and reciprocated into a crown disposed therebeneath, it is necessary that certain criteria be met: (1) that the crown be positioned as close as possible directly below the plunger, i.e. alignment must be maintained between the working surface of the plunger and the crown, and (2) that the center line of travel of the plunger be perpendicular to the surface of the crown.
If the crown is slightly out of alignment, the crown will be self-aligned by the plunger as it enters the crown. However, if the edge of the crown sidewall is beyond the lateral edge of the plunger, the crown will be crushed by the plunger. This aligning problem is generally solved by providing a centering recess in the anvil or platen on which the crown is positioned.
The second criterion involves skewness of the plungers which is caused by tolerance deviations in (1) the center line of the plunger supports, and/or (2) in the plunger shaft. The plungers are normally positioned by two vertically spaced guides in which the plunger shafts are reciprocated. When a rigid plunger enters a crown askew from the perpendicular, the plunger will only partially seal the thermoplastic material within the crown with the result that the material will flash under the unsealed portion of the plunger as it is formed, and a defective insert will be formed. Only by close tolerance machining of the guides and plunger shaft may this problem be alleviated. Since crown liner machines generally have a plurality of plungers, this part of the machine is a prime cost factor in the manufacture of crown machines. Additionally, the plunger is heated in many machines, either directly or by heat transfer from the thermoplastic material, with the result that thermal expansion may warp the plunger shaft.